(1) Field of the Invention
The present invention relates to fixing devices and image formation apparatuses, and in particular to technology of improving both mechanical strength and heat generation efficiency of a fixing member, the fixing member being included in a fixing device that uses electromagnetic induction heating, and self-adjusting the amount of heat generation.
(2) Related Art
Conventionally, there has been a known technology of reducing energy consumption of a fixing device that uses electromagnetic induction by reducing warm-up time of the fixing device and omitting preheating during the stand-by period. According to this technology, a part of fixing member with a small thermal capacity, such as a belt, is caused to generate Joule heat by electromagnetic induction using an excitation coil.
To adjust the temperature of the fixing member included in such a fixing device, a structure using magnetic shunt alloy has been proposed (Japanese Patent Application Publication No. 2010-2657). Specifically, the above publication proposes providing the fixing member with a primary heat generator layer (induction heat generator layer) made of copper (Cu) and a heat generation control layer made of magnetic shunt alloy. At or below the Curie temperature, the heat generation control layer serves as a ferromagnetic. The heat generation control layer attracts the magnetic flux generated by the excitation coil and concentrates the induced current (i.e. eddy current) to the primary heat generator layer so that the primary heat generator layer efficiently generates Joule heat.
Above the Curie temperature, the heat generation control layer serves as a paramagnetic, and passes the magnetic flux generated by the excitation coil. Hence, the magnetic flux density within the primary heat generator layer is decreased, which leads to reduction of the amount of heat generation. The magnetic flux passing through the heat generation control layer is led toward the core bar serving as a magnetic flux suppression layer. This structure prevents the non-sheet conveyance region from being overheated when small sheets are sequentially conveyed (i.e. self-adjustment of the amount of heat generation).
Permalloy (Fe—Ni alloy) is one commonly-used example of magnetic shunt alloy having a Curie temperature close to the fixing temperature and changing greatly in its magnetic permeability. Permalloy, however, does not have a high mechanical strength, and there is a problem that a fixing member using permalloy is likely to be damaged.
In addition, permalloy essentially requires annealing to obtain preferable magnetism. If annealing is performed on the entire fixing member containing permalloy, the mechanical strength of copper contained in the induction heat generator layer will be degraded as well as the mechanical strength of the permalloy, and it is impossible to obtain the strength required by a fixing member for a fixing device.
Although there is an option to first perform annealing on permalloy alone and then to form the heat generator layer by electrolytic plating, annealing forms a hard oxide layer on the surface of permalloy, and it is therefore difficult to obtain necessary peel strength between the heat generator layer and the permalloy.
Another option is to first perform annealing on a multilayer layer structure including: a layer of cladded permalloy (heat generation control layer); a layer of copper (primary heat generator layer); and a layer of Ni (antioxidant layer), and then to form a reinforcing layer by electrolytic plating. However, if the reinforcing layer is adequately formed on the surface closer to the excitation coil in order to secure the mechanical strength, the heat generation efficiency will be degraded.